For example, a nuclear power plant that includes a pressurized water reactor (PWR) uses light water as a nuclear reactor coolant and a neutron moderator while keeping the light water as high-temperature and high-pressure water which is not boiled throughout a reactor core, sends the high-temperature and high-pressure water to a vapor generator so as to generate a vapor by a heat exchange operation, and sends the vapor to a turbine generator so as to generate electric power.
In such a nuclear power plant, there is a need to periodically inspect various structures of the pressurized water reactor in order to ensure sufficient safety or reliability. Then, when a problem is found after various inspections, a necessary portion involved with the problem is repaired. For example, in the pressurized water reactor, a nuclear reactor vessel body is provided with a plurality of instrumentation nozzles penetrating a lower end plate. Further, each of the instrumentation nozzles is formed so that an in-core instrumentation guide pipe is fixed to the upper end thereof inside the reactor and a conduit tube is connected to the lower end thereof outside the reactor. Then, a neutron flux detector capable of measuring a neutron flux is insertable from the instrumentation nozzle to a reactor core (a fuel assembly) through the in-core instrumentation guide pipe by using the conduit tube.
The instrumentation nozzle is formed in a manner such that an in-core instrumentation cylinder formed of nickel base alloy is fitted into an attachment hole of a nuclear reactor vessel body formed of low-alloy steel and is welded by a material of nickel base alloy. For that reason, there is a possibility that a stress corrosion crack may occur in the in-core instrumentation cylinder due to the long-term use. Thus, when the stress corrosion crack occurs, there is a need to repair the instrumentation nozzle. A nozzle repair method of the related art is disclosed in, for example, Patent Literature 1 below. A method of repairing an elongated housing disclosed in Patent Literature 1 includes cutting an elongated housing such as a neutron flux monitor housing fixed to a lower end plate of a nuclear reactor vessel by welding at a welding portion in the vertical direction, removing the cut housing, removing a groove-welding portion for a nozzle of the nuclear reactor vessel along with the remaining housing, restoring a grooving portion to a head portion of the nozzle, fixing a housing inserted from a penetration hole of the nuclear reactor pressure vessel through a groove-welding portion for the nozzle, and fixing an inserted front end to the housing by welding.